First of all, read the corresponding wiki page. It contains all the info you need:
A quick summary:
- Make sure your add-on is free, legal to distribute, open-source and contains a license file. No .pyo files or other binaries in our official repository !
- For ease of development for both yourself and other authors who want to submit patches (bugfixes or features), try to make a git or svn repository somewhere. Github or googlecode are good choices.
- Register with the Add-ons Mailing List. This is the place to submit your work and also the place where other authors will contact you with pull requests for translations and patches. See the wiki page for more details and/or just watch the mailing list for a day or so, to get a feeling on how things work.
- Hack away !